Pulp purification and chemical recovery



Feb. 11, 1958 A. N. PARRETT PULP PURIFICATION AND CHEMICAL RECOVERY Filed Feb. 14, 1952 &

& mmmz .EQEm 5 o 7 7 6 6 5 PERCENT OF TOTAL SODIUM AS SODIUM SULFI TE I ATTO R N EY5 United States Patent PULP PURIFICATION AND CHEMICAL RECOVERY Arthur N. Parrett, Shelton, Wash., assignor to Rayonier Incorporated, Shelton, Wash., a corporation of Delaware Application February 14, 1952, Serial No. 271,621

18 Claims. (Cl. 92-11) centrated by evaporation and burned to produce a smelt which is treated for the recovery of chemicals. The invention provides a cyclic process of pulp refining, and recovery of refining chemicals with a relatively small loss of sodium chemicals, with economies in production and with a minimum of pollution.

It is now common practice in the production of highalpha cellulose to subject acid sulfite pulp which has been chlorinated and washed to digestion with caustic soda liquor at temperatures usually not in excess of 135 C. and generally below 100 C. My invention produces superior pulp having better brightness, a higher alpha cellulose content and in better yield. The pulp is further characterized by better acetylation properties, as shown by more complete and rapid dissolving in acetylation; and by better properties in the viscose process as shown by giving yarn or cord of better fatigue properties. It has not been common practice in the production of refined pulp by the sulfite digestion of wood to recover chemicals by the evaporation and binning of efiluents, neither as regards the acid sulfite digestion eflluent nor as regards the alkaline refining efiluent. Efiluents from the alkaline refining have not hitherto been burned to recover chemicals because, in contrast to conventional kraft operation, the ratio of dissolved organic wood components to inorganic chemicals is so low that there would be little net heat value to recover. Thus, the process would not be economical in view of the large capital investment and considerable operating costs for lime burning and causticizing equipment which would be required to convert the sodium carbonate produced in the furnace to the caustic soda commonly used in alkaline refining.

My invention includes the improved method of digesting a pulp, which has been produced by acid sulfite digestion of wood and subjected to chlorination and washing, with a digestion liquor consisting principally of sodium carbonate and sodium sulfite described and claimed in my copending application, Serial No. 269,659, filed February 2, 1952.

In accordance with my invention, I subject pulp from the acid sulfite digestion of wood. to digestion with a mixture of sodium carbonate and sodium sulfite in which the sodium carbonate is in an amount sufficient to sub stantially remove hemicellulose. so as to produce highalpha cellulose, and the sodium sulfite is in an amount,

2,823,120 Patented Feb. 11, 1958 In the sulfite pulping process, a lignocellulose material is generally cooked in a solution of sulfurous acid, part of the sulfurous acid being combined as bisulfite. The cation combined with the bisulfite ion is generally known as the pulping base and this term is used in this sense herein. Pulping-base cations normally used include calcium, sodium, ammonium, and magnesium. Pulp produced by acid sulfite digestion of wood, irrespective of the pulping base, is commonly termed sulfite pulp and the term is used with this meaning herein.

For application of the improvel alkaline purification treatment, the prior sulfite digestion of wood is not limited to any particular method or conditions, such as choice of pulping base. Likewise, the chemical properties of the pulp obtained from the sulfite digestion are not critical and any conventional sulfite digestion treatment may be used. Normally, however, the digestion will be so carried out that a screenable pulp will be obtained which may be chlorinated without uneconomical consumption of chlorine. Such sulfite digestion conditions will normally produce yields of 40% to of unrefined pulp, based on the dry initial wood.

Sulfitev cooking liquors to produce a screenable pulp will generally contain about 0.5% to 1.5% of combined sulfur dioxide, representing combination as neutral sulfite, and 4% to 10% of free sulfur dioxide.

Similarly, the invention is not limited to the use of any particular wood in the sulfite digestion. Any soft or hardwood which can be satisfactorily pulped by the acid sulfite process may be used. Suitable woods include western hemlock, spruce, southern pine, black gum, birch, maple, and ash.

Following sulfite digestion, the pulp is washed in any conventional or otherwise convenient manner such as inv a blowpit or by drum washers. The washed pulp in aqueous suspension is then chlorinated with elemental chlorine according to conventional practice for sulfite chlorinated and washed pulp is again digested ata temperature of at least 140 C., in a liquor containing a mixtime of sodium carbonate and sodium sulfite, of which the sodium sulfite represents 10% to 70% of the sodium present in the mixture. In an especially preferred range of compositions, sodium sulfite would represent 20% to 50% of the sodium present in the mixture.

With less than 10% of the total sodium represented by sodium sulfite, the pulps will generally be unsatisfactory with regard to brightness. With more than 70% of sodium represented by sodium sulfite, unsatisfactory alpha purification and undesirably high solubility in the standard hot potassium hydroxide solubility test (KOH solubility) will result. Moreover, such an excessive proportion of sodium sulfite will cause difliculties in chemical recovery, especially as regards the smelting operation, mainly by increasing the fusion temperature of the smelt.

Most frequently, the alkaline digestion will be carried out at a temperature above C., and, in batch operations, at temperatures below 185 C. With a digestion time of the order of 45 minutes, optimum results will generally be obtained at -175 C. With shorter times, higher temperatures may be used, even above C. The operation may be conducted batchwise or continuously in any suitable equipment. For continuous operation, shorter times and higher temperatures may be used advantageously.

The consistency of the pulp slurry is not critical, but for economical reasons will be as high as can be conveniently mixed with the mechanical equipment available. A consistency of to 20% will generally be used. The term consistency, as used herein, refers to the weight of bone dry pulp per total weight of slurry in which it is contained, expressed as percent.

The total amount of sodium carbonate and sodium sulfite used will be determined by the characteristics desired in the refined pulp. Usually from 6% to 20% of sodium salts, calculated as Na O and based on weight of bone dry pulp, will be used, more extensive refining being obtained with the higher proportions of chemicals. Since the chemicals are readily recovered, it is econom ically practical to use higher proportions of chemicals to pulp than has been possible in past sulfite pulp refining procedures using sodium hydroxide (NaOH), where the sodium hydroxide could not be practically recovered.

The highly satisfactory refining in the invention at temperatures higher than those hitherto considered desirable may be due to the highly buttered character of the refining liquor. Use of the sodium carbonate alone under these temperature conditions would suitably remove hemicellulose from the unrefined pulp. The color of the resulting pulp, however, would not equal that of pulp from customary sodium hydroxide refining and would be so dark as to be unacceptable. By using a mixture of sodium carbonate and sodium sulfite, the advantages of a buffered solution for removing hemicellulose are retained while the highly colored materials are simultaneously removed. Presumably this is due to a specific effect of the sodium sulfite on colored material present prior to, or formed during, the alkaline digestion.

While the sodium carbonate-sodium sulfite mixture used in the digestion refining of my invention gives results superior to those of the customary sodium hydroxide liquor used, it is not necessary that the digestion liquor be entirely free from sodium hydroxide. In the cyclic process of pulp refining and recovery of refining chemicals of the invention, the loss of the sodium chemicals is small. This loss, which normally should not. exceed may, if desired, be replaced by sodium hydroxide. Thus, in addition to the sodium carbonate and sodium sulfite, up to 15% of the sodium of the soda chemicals used may be added as sodium hydroxide with out requiring additional chemical recovery equipment and without necessitating expensive causticizing and lime burning equipment.

retain its highly bufiered character and will give substantially the previously described advantages in refining. There will frequently be present, in addition to the above described efiective sodium compounds, substantial amounts of sodium sulfate and smaller amounts of other sodium salts such as sodium chloride and sodium thiosulfate. These additional sodium chemicals, when present, are relatively inert and their presence is not considered in computing the effective ranges for the active chemicals, neither as regards the range of proportions of chemical composition nor asregards range of the ratio of total amount of active chemicals to pulp.

The accompanying drawing illustrates graphically the results of refining digestions of acid sulfite pulp with liquors containing varying amounts of sodium carbonate and sodium sulfite.

The following examples and tables illustrate the method and improvements of the invention:

EXAMPLE I Three batches of chlorinated and washed acid sulfite pulp from western hemlock wood were refined by digesting for 45 minutes in an agitated pot at temperatures of 125 C., 150 C., and 165 C. with liquor equivale to Also, with up to 15% of the soda salts present as sodium hydroxide, the solution will still 19.4% Na O (25% NaOH) consisting of a mixture in which one-half the Na O was Na CO and one-half-was Na SO Following the refining digestions, the pulp batches were removed, washed with water, and dried.

Alakline refining would normally be followed by bleaching. A bleaching step was, however, omitted in these tests in order to show more definitely the direct effect of the alkaline refining.

The refined and dried pulps were analyzed for alpha cellulose and tested for brightness'by the following methods which are similar to those known and used in the art:

Alphacellulose.--Fraction of pulp insoluble in 18.0% NaOH solution, by a method closely similar to TAPPI Standard Method T203m.

Brightness-Percent reflectance of light at 457 millimicrons using a Beckman Model D. U. spectrophotometer equipped with reflectance attachment. Calibration of standards was such as to give results'closely equivalent to those of TAPPI Standard Method T217m for measuring brightness with the General Electric reflection meter;

Using the above refining procedure and conditions and testing methods, the experimental values shown in Table I below were obtained:

.In order to demonstrate the improved acetylation properties of pulp refined by the invention, the following convenient and rapidlaboratory test was used for comparing the acetylation reactivity of samples of pulp fibers:

Small specimens of the pulps to be examined are moistened with distilled water and dried in a circulating oven at a controlled elevated temperature to dry them under comparable conditions. (This step may be omitted if all samples have been previously dried in the same manner.) An accurately weighed sample of 0.5 gram of each pulp is torn into small bits and placed in a ml. vial. A flattened glass rod is placed in the vial through a hole-in the'cap and the vial and sample'set in a water bath at 20 C.

The acetylating mixture is prepared by mixing 2.500 grns. H 88.0 ml. acetic anhydride, and 175.0 ml. acetic acid. This mixture is unstable and should be freshly prepared every two days.

To the sample vial in the water bath- 15.0 ml. of the acetylating mixture are added from a pipette. The pulp and acid are mixed with the glass rod, which remains in the vial. The vials are stored in the water bath and the mixing repeated every 15-20 minutes. It is important to include a standard sample with each group of unknowns .and'to handle and agitate all samples alike.

As the pulp samples are acetylated by the mixture, they dissolve continuously. The time required for substantial solution to take place and the relative clarity and residual undissolved fibers at the time of observation will indicate whether any of the samples. is more or less reactive than the standard. An observation of color is also made.

As an example of pulp refined by the invention, a sample selected from the refining experiments described in Example I was tested by the above test- This sample was the one refined at C. with' a 50-50 mixture of Na CO and Na SO In the above described acetylation test, it dissolved in 7 hours to give a clear 'solu-' 3 EXAMPLE 1v A sample of chlorinated hemlock sulfite pulp was refined by digesting it 45 min. at 170 C. in a liquor containing sodium chemicals in the proportion of 10% Na o fibers EXAMPLE H on the pulp basis, the sodium chemicals being present as I Na CO 55%, Na SO 35%, and NaOH This pulp In Table II are shown comprehensive analytical and was bleached with sodium hypochlorite giving a product acetylation test data on a series of samples of western with the following analytical properties: Alpha 96.7%, hemlock sulfite pulp refined at 160 C. with sodium car- 10 Brightness 89.0%, KOH solubility 3.3%. The pentosan bonate-sulfite mixtures in amount in each case equivcontent was low (1.0% as determined by distillation alent to 19.4% Na O (equivalent to 25% NaOH) on with hydrobromic acidand precipitation with barbituric the pulp. The proportions of Na CO and Na SO are U- however varied from 100% Na CO to 100% Na SO The bleached pulp was made into viscose contain- All proportions are on the basis of equivalents of Na O. ing 7.5% cellulose and 6.5% NaOH by a conventional The analytical and test methods used, in addition to standard procedure and spun into a tire cord using 21 those already described in Example I, included the destandard procedure customarily used for evaluating pulps. termination of KOH solubility by measuring the frac- The rayon cords were tested for their breaking strength tion soluble on heating the pulp three hours at 100 C. in both the dry and wet state, their degree of elongation in 10% KOH solution. at break in the dry state, and their fatigue life, the last The results obtained with the refining test methods of test being made on a machine having a reciprocating Example 11 follow in Table II: motion, with the cord maintained at a temperature of Table II Refining Agent Pct. Acetylatlon Test of N 920 as- Reflning Alpha KOH Bright- Yield, Cellulose, Soluble, ness, Percent Percent Percent Percent Soln Clarity of Color of Na C 0; N21230: Time, Soln Soln 100 0 79. 6 96. s a. 9 54.3 6 90 10 7s. 5 96. 3 4. a 56. 5 6 70 .30 79. 6 96. 3 5. a 64. 0 6% 50 50 79. 6 96. 7 4. 5 68. 4 6% 70 as. 0 94. 9 9. 2 7o. 2 6% 10 90 86. o 94. 2 9. 9 74. 4 7 0 100 87. s 94. 1 10. 9 76. 1 7

fi i g. i emghasize the efiects oflthe Varying 150 C. and under a load of 1.81 kilograms. These tests mug con monst e abqve analyses and teats E? resulted in the following results: Tenacity, gm./denier, on refinqi a wlthout i i n dry 3.09, wet 1.98; elongation 12.1%; fatigue 302 minutes g i e pu.phwould Tbleac d o g as compared with a fatigue life of 257 minutes for a i pulp bug L Va Hes i cord from conventionally refined pulp. Moreover, the i jfifsg g gi e acetates consider? yhg ter pulp gave a yield of 98.8% regenerated cellulose as c m ared to 96.07 to conve tionall refined ul On examination of the analytical data for alpha, brighto p 0 r n y P p ness, and KOH solubility, and both dissolving time and EXAMPLE V color in the acetylation test, it will be seen that the most A f h] d lfi 1 th 4 generally satisfactory pulps were those refined within g fgz i Sou i i the preferred chemical proportion limits of the inveni g g' y rlgesnng p i q 1 tion, namely with 10 to 70% of the Na O represented by at m a quot contammg so mm c ,emlca S K sodium sulfiten proportion of 12% Na O on the pulp basis, the sodium I EXAMPLE HI chemlcals being present as Na CO Na SO 35%, and NaOH 10%. This pulp was bleached with sodium n Table III given analytlcal and acetylaflon test hypochlorite and the finished product had the following 'data similar to Table II for a Seri s f samples of h l analytical properties: Alpha cellulose 97.5%, Brightness lock sulfite pulp r fin d at In each case with 55 88.7%, KOH solubility 3.6%. The pulp Was also notable equal parts of Na CO and Na SO in terms of N320, in having a low pentosan content (1.4% as determined and with variation of the total amount of sodium sulby distillation with hydrobromic acid and precipitation fite-carbonate from 7.75% to 25.2% Na O based on with barbituric acid). pulp. The pulp was acetylated by a conventional test pro- T able Ill Acetylatlon Test Total Refining Alpha KOH Bright- N820, Yield, Cellulose, Soluble, ness, 1 Percent Percent Percent Percent Percent Solution Clarlty of Color of Time, Soln Soln Hrs.

7.8 86.7 94.8 9.6 69.8 8 Hazy Tan. 13.6 83.0 95.9 7.0 71.7 7 s1. Haze.-.. Do. 19.4 79.6 96.7 4.6 68.4 7 o Do. 25.2 82.2 95.8 7.1 71.7 7 do Do.

Table III shows that satisfactory results were obtained over a wide range for the ratio of total chemical to wood but that optimum results were obtained with .an amount of 50- 50 sodium sulfite-carbonate equivalent to about 19%"Na;0'. i

cedure in which it reacted smoothly to give an acetate of good clarity, color and filterability.

EXAMPLE VI In order to show what the effect would be in practice of recycling efiiuent liquor from the refining of the invention to build up its content of organic materials, a series of successive alkaline refining digestionsiwas made'on separate samples of hemlock sulfite pulp. In each case, the alkaline digestion liquor was made up by adding sodium chemicals in the proportions of 55% Na CO Na SO and 10% NaOH. The alkaline digestions were made at 170 C. for minutes at a consistency of 10% and using 10% total sodium chemicals as Na O based on the pulps.

In carrying out the series, however, after the first digestion in fresh liquor, all the efliuent digestion liquor that could be removed from the refined pulp by centrifuging was used in making up the liquor for the next digestion and similarly through the suceeding digestions. In each case, water and chemicals were added to give the required volume of digestion liquor containing fresh Na- CO Na SO and NaOH in amount equivalent to 10% Na O on the pulp and in the proportions stated. In addition, the solution contained small amounts of residual active chemicals from the liquor recycled from the previous digestion.

Each of the successive digestions provided refined pulp of satisfactory quality and with no retrogression of quality due to recycling other than a slightly higher bleach demand.

After the fifth and final digestion, the efiluent liquor Was found to contain 21.3% solids and was evaporated under vacuum to approximately 47% solids.

The evaporated solution then had the following analysis:

The B. t. u. value of the solids indicates that it probably would be necessary to use an auxiliary fuel such as fuel oil along with this liquor to maintain suitable combustion. The sodium and sulfur analysis, however, correspond to a sulfidity of about 31% which indicates that combustion would produce a low melting smelt. This is very important from the standpoint of ease and simplicity in carrying out the furnacing operation.

Table IV illustrates the variations in fusion temperatures of various mixtures of Na s and Na CO In column 3, the mixtures of column 1 have been calculated to express the total Na O present as Na S, thus expressing the proportions on a basis comparable with the proportions of Na SO in the Na CO Na SO compositions of my invention. It will be noted that under reducing conditions in the furnace, neglecting any loss of sulfur in the stack gases, one molecule of Na SO will theoretically give one molecule of Na S in the smelt.

Table IV Percent of Total N azO Present as Nags Percent Na S in N ans-N e200; Mixture In actual smelting operations,

the smelt will contain "8 appreciable amounts of sodium sulfate-and minor amounts of other sodium salts. These will tend to lowersomewhat the fusion points given above for mixtures of pure sodium carbonate and sodium sulfite. I

In one sample of smelt in which the percentage of the total Na O present as Na s was 45.0%, the Na CO 45.0% and Na SO 10.0%, the fusion point was 738 C. In another sample of smelt in which the percentage of the total Na O present as Na S was 70.0%, the Na CO 20% and Na SO 10.0%, the fusion point was 777 C. ollowing digestion, the liquor associated with the pulp may be recovered from the pulp with a minimum of dilution, as, for example, by displacement on a rotary drum washer. A substantial portion of this effluent liquor containing residual amounts of active chemicals and organic iatter may be recycled directly to the digestion liquor makeup. This is advantageous from the standpoint of building up the dissolved organic material.

The remainder of this effluent liquor containing a high I content of organic matter by virtue of recycling can be evaporated with substantial economy. Evaporation may be in any conventional manner, as, for example, in a multiple effect vacuum evaporator, followed if desired by a direct contact with flue gas in a cascade evaporator. Depending on viscosity and equipment characteristics, the liquor will generally be evaporated to a solids content of 50%75%.

The evaporated liquid is then burned in a smelting recovery furnace under reducing conditions. The furnace will generally be equipped with water tubes to recover heat and generate steam and may be a conventional kraft type recovery furnace.

Due to the relatively high ratio of inorganic to organic solids, it may be necessary to supply auxiliary fuel such as fuel oil. This, however, cannot entirely be considered as an additional cost since the fuel value of this additional oilas well as a substantial portion of that of the organic matter in the liquor will be recovered in the steam generated.

Due to its high carbonate content, the smelt produced by the process of the invention has a relatively low melting point. Thus, it flows freely from the furnace at moderate temperatures, avoiding excessive fume formation and excessive recovery costs. Furnace operation will be especially favorable in preferred cases where the ratio of sodium sulfite to total soda chemicals (all expressed as Na O) in the liquor used for the alkaline pulp digestion does not exceed 50%.

Where an auxiliary fuel is required, concentrated sulfite waste liquor may be used if desired in place of fuel oil. The concentrated sulfite Waste liquor, however, should be either sodium or ammonium base liquor. This may result from digesting the wood with a solution containing sulfurous acid and either sodium or ammonium bisulfite. Alternatively, where calcium base acid sulfite wood digestion liquor is employed, the waste liquor may be readily converted into suitable sodium base form, as, for example, by contacting the liquor with cation exchange resin of the sulfonic acid type in either the sodium form or as a mixture containing both sodium and hydrogen forms.

Use of sulfite waste liquor as an auxiliary fuel has a 7 number of advantages including economy of fuel, in-

creased heat recovery, minimizing of pollution and recovery of additional chemicals. However, it will be noted that use of sulfite waste liquor will increase the ratio of sulfur to sodium in the liquor and in extreme cases will necessitate considerably higher furnace operating temperature due to the higher sulfidity of the smelt.

The smelt will. consist largely of sodium carbonate and sodium sulfide, together with smaller amounts of sodium sulfate, sodium thiosulfate, traces of sodium sulfite, and generally some sodium chloride as an impurity.

From this smelt the original alkaline digestion chemicals (Na CO and Natmay be economically regenerated. -While the invention is not limited to any specific method of converting the smelt to alkaline digestion chemicals, this may be readily and practically accomplished by the solution carbonation process of pending application, Serial No. 415,817, filed March 12, 1954, of Kenneth Russell Gray, Hartzell Lance Crosby and John Charles Steinberg. In this process the smelt is dissolved and then, without any solids handling steps, treated to convert the sulfide content substantially to carbonate, the sulfur being eliminated as concentrated H 8. This H 8 is readily burned to S; which, with any additional makeup, may be used to convert any desired portion of the sodium carbonate to sodium sulfite.

The fact that the alkaline refining chemicals used in this process may be regenerated from the smelt without the use of expensive lime burning and causticizing equipment, required in kraft and soda pulping recovery, is of great practical advantage.

If the sulfite wood digestion is accomplished by soda base acid sulfite liquor and the wate efiluent used as auxiliary fuel in combusting the waste alkaline refining liquor of the invention, a completely integrated cyclic process of acid sulfite pulping and alkaline refining becomes possible. Thus, by the aforementioned solution carbonation process, Serial No. 415,817, the smelt may not only be converted to sodium carbonate and sulfite as described but also to sodium bisulfite-sulfurous acid solution for use in the acid sulfite digestion of wood.

Alternatively, the invention provides a completely cyclic acid sulfite pulping and alkaline refining process in which aportion of the smelt is treated by the aforementioned solution carbonation process, Serial No. 415,817, to produce the alkaline refining chemicals (Na CO and Na SO The remaining portion of the smelt may be converted to sodium bisulfite-sulfurous acid wood digestion liquor by ion exchange according to United States Letters Patent 2,656,244 of Kenneth Russell Gray and Hartzell Lance Crosby.

1. The improvement in the production of high-alpha cellulose pulp which comprises subjecting wood chips to an acid sulfite digestion with calcium-base liquor, removing a substantial portion of the sulfite digestion eflluent, replacing the calcium content substantially by sodium through contacting the removed liquor with a cation exchange resin of the sulfonic acid type in substantially calcium-free form and at least partially in the sodium form; subjecting the resulting pulp to an alkaline refining digestion with a liquor consisting principally of a mixture of sodium carbonate and sodium sulfite, said sodium;su lfite varying from to 50% and said sodium e carbonate varying from 35% to 80% of said mixture, said solution containing a total of from 6% to 20% of said chemicals (all said percentages expressed as Na O) at a temperature above 140 C., but not above about 185 C., removing a portion of the alkaline refining liquor for makeup to digestion strength and reuse in a subsequent similar alkaline refining digestion, removing another'portion of the alkaline refining liquor and combining it with the acid sulfite digestion liquor, evaporating the combined liquors and burning the same to recover a smelt.

2. The improvement in the production of high-alpha cellulose pulp which comprises subjecting wood chips to an acid sulfite digestion with calcium-base liquor, removing a substantial portion of the sulfite digestion efliuent, replacing the calcium content substantially by sodium through contacting the removed liquor with a cation exchange resin of the sulfonic acid type in substantially calcium-free form and at least partially in the sodium form; subjecting the resulting pulp to an alkaline refining digestion with a liquor consisting principally of a mixture of sodium carbonate and sodium sulfite, said sodium sulfite varying from 20% to 50% and said sodium carbonate varying from 35% to 80% of said mixture, said solution containing a total of from 6% to 20% of said chemicals (all said percentages expressed as Na O) at a temperature above 140 C. but not above about 185 C., removing a portion of the alkaline refining liquor for makeup to digestion strength and reuse in a subsequent similar alkaline refining digestion, removing another portion of the alkaline refining liquor and combining it with sodium-containing acid sulfite liquor from the exchange treatment.

3. The method of producing bright high-alpha cellulose and recovering sodium chemicals used in the digestion in which wood pulp which had been digested with acid sulfite solution, chlorinated and washed, is given an alkaline refining treatment which comprises digesting the washed pulp with an alkaline aqueous solution consisting principally of a mixture of sodium sulfite and sodium carbonate in which the sodium sulfite varies from 20% to 50% and the sodium carbonate varies from 35 to said solution containing a total of from 6% to 20% of said chemicals (all said percentages expressed as Na O) and up to about 21% by weight of organic matter from a previous similar digestion, said alkaline digestion being carried out with a pulp consistency of from 10% to 20% and at a temperature of from to 185 C. and for a suflicient time to efiectively remove hemicellulose and colored matter thereby producing bright high-alpha cellulose in good yield, separating alkaline liquor from the alkaline digestion, removing one portion of the separated alkaline liquor and adjusting the composition of the remaining alkaline liquor to digestion strength by the addition of water and sodium chemicals, repeating the alkaline digestion with the adjusted liquor on other chlorinated and washed pulp, continuing to separate, remove, adjust the composition and repeat as aforesaid, the amount of liquor removed and the amount of sodium chemicals and water added being such that the amount of organic matter does not exceed about 21%, evaporating and burning the said removed portion of the alkaline liquor which is concentrated in organic matter producing a fused smelt, and treating the smelt for return of the sodium chemicals to the alkaline refining digestion.

4. In the method of claim 3, carrying out the operation with an alkaline solution in which the sodium compounds include a total of from 50% to 80% of sodium carbonate and sodium hydroxide both expressed as Na O.

5. In the method of claim 3, carrying out the digestion at a temperature in the range of from to C.

6. In the method of claim 3, carrying out the alkaline refining during a period of about 45 minutes.

7. The method of producing bright high-alpha cellulose and recovering sodium chemicals used in the digestion in which wood pulp which had been digested with acid sulfite solution, chlorinated and washed, is given an alkaline refining treatment which comprises digesting the washed pulp with an alkaline aqueous solution consisting principally of a mixture of sodium sulfite and sodium carbonate in which the sodium sulfite varies from 20% to 50% and the sodium carbonate varies from 35% to 80%, said solution containing a total of from 6% to 20% of said chemicals (all said percentages expressed as Na O) and up to 21% by weight of organic matter from a previous similar digestion, said alkaline digestion being carried outwith a pulp consistency of from 10% to 20% and at a temperature of from 140 to C. and for a suflicient time to effectively remove hemicellulose and colored matter thereby producing bright high-alpha cellulose in good yield, removing a substantial proportion of the alkaline liquor from the refined pulp, adjusting the composition of a predetermined portion of the removed liquor for a subsequent similar digestion by the addition of water and sodium carbonate and sodium sulfite, repeating the digestion with the adjusted liquor on other chlorinated and washed acid sulfite pulp and continuing the removing, adjusting of the composition and digestion cyclically to accumulate in the liquor the required content of organic matter but not exceeding about 21%, and subjecting another portion of the removed liquor having "1'11 accumulated organic matter to concentration and burning for the recovery of the sodium chemicals.

8. In the method of claim 7, adjusting the digestio liquor by adding, in addition to the sodium carbonate and sodium sullite, an appreciable amount of sodium hydroxide up to 9. In the method of claim 7, adding acid sulfite waste liquor to the alkaline liquor portion subjected to evapcrating and burning to increase the organic content thereof.

10. The improvement in the production or" high-alpha cellulose wihch comprises subjecting wood chips to an acid sulfite digestion with a liquor of thegroup consisting of soda base liquor and ammonium base liquor, removing a substantial portion of the sulfite liquor and subjecting the resulting pulp to .an alkaline refining digestion with an alkaline aqueous solution consisting principally of a mixture of sodium sulfite and sodium carbonate in which the sodium sulfite varies from to 50% and sodium carbonate varies from to 80%, said solution containing a total of from 6% to 20% of said chemicals (all said percentages expressed as Na O), said alkaline digestion being carried outwith a pulp consistency of from 10% to 20% and at a temperature of from 140 to 185 C. and for a suflicient time to efiectively remove hemicellulose and colored matter thereby producing bright highalpha cellulose in good yield, removing a portion of the alkaline refining liquor for makeup to digestion strength by addition of depleted chemicals and water and reuse in a subsequent similar alkaline refining, and removing another portion of the alkaline refining liquor and combining it with the acid sufite digestion liquor, evaporating the combined liquors and burning the same to recover a smelt.

11. In the method of claim 10, subjecting the acid sulfite pulp to chlorination and washing before the alkaline digestion.

12. The improvement in the production of high-alpha cellulose which comprise subjecting wood chips to an acid sulfite digestion with a liquor of the group consisting of soda base liquor and ammonium base liquor, separating the pulp from the sulfite liquor and subjecting the pulp to an alkaline refining digestion with an alkaline aqueous solution consisting principally of a mixture of sodium sulfite and sodium carbonate in which the sodium sulfite varies from 20% to and sodium carbonate varies from 35% to said solution containing a total of from 6% to 20% of said chemicals (all said percentages expressed as Na O), said alkaline digestion being carried out with a pulp consistency of from 10% to 20% and at a temperature of from to C. and for a sufficient time to effectively remove hemicellulose and colored matter thereby producing bright high alpha cellulose in good yield, separating alkaline liquor from the alkaline digestion, adjusting the composition of one portion of the separated liquor by the addition of chemicals and water to digestion strength, repeating cyclically the separating, and adjusting of the composition of the portion of the liquor and its use in digestion on other similar separated pulp to form a digestion liquor containing the required content of organic matter but not over about 21% of organic matter, combinig with a portion of the liquor thus enriched in organic matter some of the acid .sulfite digestion liquor, and evaporating the combined liquors and burning the same to recover a smelt.

13. In the method of claim 1, said alkaline digestion liquor comprising up to 15% of sodium hydroxide.

14. In the method of claim 2, said alkaline digestion.

liquor comprising up to 15 of sodium hydroxide.

15. In the method of claim 3, said alkaline aqueous solution comprising up to 15% of sodium hydroxide.

16. in the method of claim 7, said alkaline aqueous solution comprising up to 15% of sodium hydroxide.

17. In the method of claim 10, said alkaline aqueous solution comprising up to 15% of sodium hydroxide.

18. In the method of claim 12, said alkaline aqueous solution comprising up to 15% of sodium hydroxide.

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Yorston, Dominion Forest Service Bull. 97, Ottawa, Canada, pp. 32, 39, 41, 46, 48, 53, 55 and 62 (1942').

Cellulose and Cellulose Derivatives by Ott, pp. 600, 601 and 815; publ. by Interscience Publishers, Inc., N. Y. (1943) Pulp and Paper Manufacture; vol. I; pp. 254, 354, 400, 401; publ. by McGraw Hill, New York (1950). 

1. THE IMPROVEMENT IN THE PRODUCTION OF HIGH-ALPHA CELLULOSE PULP WHICH COMPRISES SUBJECTING WOOD CHIPS TO AN ACID SULFITE DIGESTION WITH CALCIUM-BASE LIQUOR, REMOVING A SUBSTANTIAL PORTION OF THE SULFITE DIGESTION EFFLUENT, REPLACING THE CALCIUM CONTENT SUBSTANTIALLY BY SODIUM THROUGH CONTACTING THE REMOVED LIQUOR WITH A CATION EXCHANGE RESIN OF THE SULFONIC ACID TYPE IN SUBSTAN-TIALLY CALCIUM-FREE FROM AND AT LEAST PARTIALLY IN THE SODIUM FORM; SUBJECTING THE RESULTING PULP TO AN ALKALINE REFINING DEGESTION WITH A LIQUOR CONSISTING PRINCIPALLY OF A MIXTURE OF SODIUM CARBONATE AND SODIUM SULFITE, SAID SODIUM SULFITE VARYING FROM 20% TO 50% AND SAID SODIUM CARBONATE VARYING FROM 35% TO 80% OF SAID MIXTURE SAID SOLUTION CONTAINING A TOTAL OF FROM 6% TO 20% OF SAID CHEMICALS (ALL SAID PERCENTAGES EXPRESSED AS NA2O) AT A TEMPERATURE ABOVE 140*C., BUT NOT ABOVE ABOUT 185*C., REMOVING A PORTION OF THE ALKALINE REFINING LIQUOR FOR MAKEUP TO DIGESTION STRENGTH AND REUSE IN A SUBSEQUENT SIMILAR ALKALINE REFINING DIGESTION, REMOVING ANOTHER PORTION OF THE ALKALINE REFINING LIQUOR AND COMBINING IT WITH THE ACID SULFITE DIGESTION LIQUOR, EVAPORATING THE COMBINED LIQUORS AND BURNING THE SAME TO RECOVER A SMELT. 